1. Preheat oven to 180Â°C (conventional). Grease a deep, round 22cm cake tin with butter and line the base with non-stick baking paper.
2. Place cocoa in a small bowl and gradually stir in the boiling water with a fork until the mixture is smooth. Set aside to cool.
3. Use electric beaters to beat the butter, sugar and vanilla essence until pale and creamy. Add the eggs one at a time, beating well after each. Beat in the cocoa mixture on low speed. Use a large metal spoon to fold in the flour, then the milk. Spoon into the prepared tin and smooth the surface. Bake in preheated oven for 55-60 minutes or until a skewer inserted in the centre comes out clean. Leave in the tin for 5 minutes, then turn onto a wire rack to cool.
4. To make the frosting, sift the icing sugar and cocoa together into a mixing bowl. Add the butter and water, and use electric beaters to beat until creamy. Spread the frosting over the cooled cake.